Author Topic: Rear gear with a 3 speed  (Read 910 times)

Offline retroboy

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Rear gear with a 3 speed
« on: July 14, 2018, 10:56:30 PM »
Just curious to know what rear gears guys with a 3 speed trans are running in a front engine car? I posted this some where else and started a small xxxx fight about why I couldn't or shouldn't use a 3 speed. So if you don't have something more than (in your opinion) I need a 'glide I'm not really interested.

I have a mild injected SBM with a TF 904 (no they don't explode and no they aren't heavy) with a 2.45 first gear and an 8" converted. The car is short at 155".  I have a 4.3 rear with 31" tyre and have to fight with the car off the line. I'm looking to go back to 3.89/3.9 rear gear to soften the launch. Any thoughts on that sort of number?
 
Cheers

Offline fuel749

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Re: Rear gear with a 3 speed
« Reply #1 on: July 15, 2018, 09:33:31 AM »
I saw the other post you mentioned but it only had a couple replies when I lost track of it.  I didn't reply to it and I'm hesitant to reply to this so keep in mind this is just my opinion. I'm not telling you you shouldn't or can't run a 3 speed.  Some folks do and do okay with it but lot's of others ask the same question you are for the same reason.

     In my opinion, you have to much low gear.  It seems like you're onto that by wanting to change the rear gear. You might want to look at your starting line ratio.  Here's a couple for your combo:

4.30 gear/2.45 low= 10.54 SLR
3.90 gear/2.45 low= 9.56   SLR

Now here's a couple for a commonly used transmission with a stock low gear:

4.30 gear/1.76 low = 7.57 SLR
3.90 gear/1.76 low = 6.86 SLR

     You can see that the low (trans) gear ratio makes a bigger change than the rear gear change.  You'd have to drop to a 3.20 gear to get close to  the same SLR as the 4.30/1.76 combo which would have you crossing the finish line at a lower RPM than I'm assuming you'd like. Essentially numbing up your car and taking away performance that I'm sure you'd like to see on the scoreboard.

    So what does all this SLR stuff mean?  A lower SLR is good for lighter cars even with moderate power.  A higher SLR is needed to get heavier cars off the line.  But then again a heavy car with a ton of power generally needs a lower SLR.

   I would never tell anybody what to do but this is my opinion and it's backed up with real information.  Don't take it from me as fact, research it yourself and draw your own conclusions.

Offline Calkins

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Re: Rear gear with a 3 speed
« Reply #2 on: July 15, 2018, 10:24:49 AM »
There is a company selling TH400 transmissions that run as a two speed.  Maybe it is just a kit?  I believe they use second and third gears.  Maybe that would work for the TF727?

I am in the same boat, I want a C4 behind my Y Block Ford engine.  Call it stubbornness or call is stupidity.  But, I want a Ford powered Ford with Ford parts.  I will give up performance for the pursuit of happiness.

As my favorite quote says;  A chump like me will always choose passion over common sense...
Justin Calkins - Iowa Falls, Iowa

Offline Roger

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Re: Rear gear with a 3 speed
« Reply #3 on: July 15, 2018, 10:39:29 AM »
Have you tried leaving the line in 2nd gear and see how it reacts? Back in the 70s my first fed came with a 350 Turbo trans. and convertor that I put my engine into. I took it to my first AHRA (remember them?) national event and in my youthful excitement I made a time trial pass with the trans. mistakenly in 3rd gear. It ran 3/10s quicker using high gear only than my best ever previous pass using all 3 gears. So the next and last time trial I started in 2nd and shifted into 3rd. Ran the next to the quickest time, running about 2/10s quicker. Starting in 2nd will probably soften the launch on your car Id think but would need the torque convertor stalling at about max torque to be really efficient. Would probably work better with the later style 904 with the 1.54 second gear ratio. My opinion for what its worth:)

Offline retroboy

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Re: Rear gear with a 3 speed
« Reply #4 on: July 15, 2018, 12:21:27 PM »
Thanks for the replies. I have always believed in a SLR range of 9 - 11:1 which is where I am now. Going to 3.9's still keeps me there while 3.7's drop me just under. When I came back to racing after many years away I was working with a guy who ran a C4 trans with 3.7 gears with a little 28.5" tyre running off the brake. As far as leaving in Second gear I tried it but knocked four tenths off. But you've reminded me something I'd forgotten which was I was going to see if the transbrake would work in second gear?
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Offline jeff/21

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Re: Rear gear with a 3 speed
« Reply #5 on: July 15, 2018, 01:40:30 PM »
we ran a 4.86/1.76 and 33"x 10.5 tires tried 4.30 and 4.10 and slowed down our SLR 8.55 with TALL tires small blocks need rpm
in your case as stated above try 2nd gear starts you need the rpm down the track.
question how big is your SBM?

Offline retroboy

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Re: Rear gear with a 3 speed
« Reply #6 on: January 02, 2019, 03:16:22 AM »

I am in the same boat, I want a C4 behind my Y Block Ford engine.  Call it stubbornness or call is stupidity.  But, I want a Ford powered Ford with Ford parts.  I will give up performance for the pursuit of happiness.

As my favorite quote says;  A chump like me will always choose passion over common sense...

I Mate of mine ran a very light slingshot with a basic 302 and C4.  Had a 3.7 rear and 28.5 tyre. Used to just glide the fronts an inch or two of the ground. You could only see it in a pic not with the eye so no probs with ya Y Block.

Cheers

Offline jeff/21

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Re: Rear gear with a 3 speed
« Reply #7 on: January 02, 2019, 07:27:27 AM »
the car was 160" heavy 1700 lbs had a rad water pump and battery if the motor is built to turn 5-6-7-8 or 9000 rpm i like it to turn that rpm just before the finish in high gear

Offline jeff/21

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Re: Rear gear with a 3 speed
« Reply #8 on: January 02, 2019, 10:31:41 AM »
i would like to add  that it isn't always possible especially in 1/8 to the top of 2nd 3rd by changing trans gear first choice or tire size or rear gear than up shifting and going through lower rpm